Historically medical practice for the measurement of hemodynamic blood flow parameters has been based on the use of a pulmonary artery catheter. This device is highly invasive and requires a catheter to be introduced through a large vein—often the internal jugular, subclavian, or femoral vein. The catheter is threaded through the right atrium and ventricle of the heart and into the pulmonary artery. The standard pulmonary artery catheter (Swan-Ganz) has two lumens (tubes) and is equipped with an inflatable balloon at the tip, which facilitates its placement into the pulmonary artery through the flow of blood. The balloon, when inflated, causes the catheter to “wedge” in a small pulmonary blood vessel. So wedged, the catheter can provide a measurement of the blood pressure in the left atrium of the heart, termed Left Ventricular End Diastolic Pressure or LVEDP. Modern catheters have multiple lumens (multiple tubes) five or six are common and have openings along the length to allow administration of inotropes and other drugs directly into the right atrium of the heart. The addition of a small thermistor temperature probe about 3 centimeters behind the tip allows the measurement of blood flow following calibration by means of the injection of a known volume and known temperature of cold fluid. As this cooler fluid passes the thermistor, a brief drop in the blood temperature is recorded. The resulting information can be used to compute and plot a thermodilution curve. If details about the patient's body mass index, core temperature, systolic, diastolic, central venous pressure and pulmonary artery pressure are input, a comprehensive blood flow and pressure map can be calculated. The procedure is not without risk, and complications can be life threatening. It can lead to arrhythmias, rupture of the pulmonary artery, thrombosis, infection, pneumothorax, bleeding etc. The benefit of the use of this type of catheter device has been controversial and as a result many clinicians have limited its use.
Current hemodynamic monitoring systems rely on one of a variety of technologies namely; ultrasound Doppler flow measurement; arterial pressure utilizing mathematical algorithms to derive blood flow; electrical signals from skin electrodes; rebreathing pulmonary gas exchange; and mechanical accelerometry.
The measurement of vascular blood flow by use of ultrasound and the Doppler principle is widely used. A probe containing piezo-electric crystals driven to emit either continuous wave or pulse wave ultrasound is located close to an arterial blood vessel. The probe may be located either in the esophagus, trachea or is placed on the body surface in a position where an artery can be accessed such as at the suprasternal notch. The velocity of the blood flow is calculated using the Doppler equation:
  v  =            c      ·              f        D                            2        ·                  f          T                    ⁢      Cos      ⁢                          ⁢      θ      
where v is the velocity of the red blood cells, c is the speed of the ultrasound waves through body tissues, fD is the Doppler frequency shift, fT is the transmitted frequency of the ultrasound and Cos θ is the cosine of the angle of insonation between the sound beam axis and the direction of blood flow. Flow based measurements using ultrasound are well suited to use in hemodynamically unstable patients where rapid and frequent changes in blood flow and pressure are encountered. An ultrasound beam directed into the vascular system provides accurate real time measurement of blood flow. However, the ultrasound beam is directional and is sensitive to movement, which requires the operator to check the beam's focus and thus the device cannot be considered to be providing continuous monitoring without operator attendance.
A second approach utilizes pressure measurements of arterial blood pressure measured invasively through an arterial cannula placed in an artery usually radial, femoral, dorsalis pedis or brachial. The cannula must be connected to a sterile, fluid-filled system, which is connected to an electronic pressure transducer. The advantage of this system is that pressure is constantly monitored beat-by-beat, and a waveform (a graph of pressure against time) can be displayed. Cannulation for invasive vascular pressure monitoring may be associated with complications such as extravasation, thrombosis and infection. Patients with invasive arterial monitoring require very close supervision, as there is a danger of severe bleeding if the line becomes disconnected. There are a variety of invasive vascular pressure monitors these include single pressure, dual pressure, and multi-parameter (i.e. pressure/temperature). Vascular pressure parameters such as systolic, diastolic and mean arterial pressure are derived and displayed simultaneously for pulsatile waveforms. Such devices utilize pulse contour or pulse pressure wave analysis where the area under the systolic pressure wave curve is integrated or wave characteristics are analyzed, and when calibrated against either dye dilution or thermodilution, provide estimates of blood flow volume. These systems are known to have good correlation to blood flow based measurements in hemodynamically stable patients as assessed by comparison to measurements made by a pulmonary artery catheter. However, these devices are known to be problematic in monitoring rapid changes in patients who are hemodynamically unstable. These devices can lead to erroneous cardiac output measurements during the administration of vaso-active drugs, during loss of circulating volume e.g. hemorrhage, insufflation of the abdomen for laparoscopic surgery, pathophysiological diseases resulting in abnormal arterial pressure waves and positional changes during surgery. Drugs which create vasoconstriction result in an increase in systemic resistance and thus increase in pressure which is interpreted as an increase in flow. Whereas blood flow typically decreases as systemic resistance increases as the heart is acting to pump against increased resistance. Conversely drugs which have a vasodilation effect result in a decrease in resistance to blood flow and typically blood pressure falls which is interpreted as a reduction in flow. Whereas blood flow typically increases as systemic resistance decreases as the heart is acting to pump against a reduced resistance. Calibration is essential for absolute value accuracy and in operating room conditions such calibration is complex to perform, is time consuming, needs to be repeated frequently, introduces chemical agents which may be toxic and may be of limited accuracy in the presence of other drugs administered during patient treatment.
Thus, there is a need in the art for an improved method and apparatus for hemodynamic monitoring using combined flow and pressure measurement information, which overcomes one or more of the aforementioned drawbacks.